1. Field of the Invention
The present invention relates to an imaging apparatus, an imaging apparatus control method, and a computer program. More specifically, the present invention relates to an imaging apparatus, an imaging apparatus control method, and a computer program that make it possible to perform accurate auto-focus processing for a target subject.
2. Description of the Related Art
Recently, an auto-focus (AF) mechanism that automatically takes the focus on a subject is mounted on many imaging apparatuses such as a still camera and a video camera. In photographing by general cameras, people are main subjects in a large number of cases. However, the people are present in various positions in photographed images and there are various compositions of the images. From the very beginning, a target subject may be present in a position where a range finding area in an imaging apparatus is not set. To cope with such a problem, it is proposed to recognize a face from an image in an imaging apparatus and set a range finding area in a position of the recognized face to make it possible to focus the imaging apparatus on the face regardless of a composition. For example, a focus control technique based on such face recognition is disclosed in JP-A-2003-107335. Focus control based on this face recognition makes it possible to perform appropriate focus control (focusing) in photographing of various compositions.
As a method of focus control, a method of judging a level of contrast of imaging data acquired via a lens is known. This is a system for setting a specific area of a photographed image as a signal acquisition area for focus control (a spatial frequency extraction area), judging that the lens is more in focus as contrast of this specific area is higher and that the lens is out of focus when the contrast is low, and driving the lens to a position for increasing the contrast to adjust the focus. Such a focus control processing technique is disclosed in, for example, JP-A-10-213737.
Specifically, a method of extracting a high-frequency component of the specific area, generating integrated data of the high-frequency component extracted, and judging a level of contrast on the basis of the high-frequency component integrated data generated is applied to the method of focus control. In other words, plural images are acquired while a focus lens is moved to plural positions and filter processing represented by a high-pass filter is applied to a luminance signal of each of the images to obtain an AF evaluation value indicating contrast intensity of each of the images. In this case, when a just-focused subject focused in a certain focus position is present, an AF evaluation value corresponding to a focus lens position draws a curve shown in FIG. 1. A peak position P1 of this curve, i.e., a position where a contrast value of the image is the maximum is a just-focus position. In this system, it is possible to perform a focus operation on the basis of only information of an image displayed on an imager and it is unnecessary to provide a range-finding optical system other than an imaging optical system. Thus, this system is widely used in digital still cameras recently.
In this way, in the auto-focus control, a high-frequency component integrated value of the specific area is calculated for judging the level of the contrast and used as an evaluation value (an AF evaluation value). Auto-focus is realized by driving the focus lens to maximize the evaluation value. To perform the auto-focus, it is necessary to drive the lens with the evaluation value as a guideline. As a lens driving mechanism, for example, a voice coil motor is used.
However, when another person cuts in front of a subject set as a focus object during such a focus operation, a problem described below occurs. This problem will be explained with reference to FIGS. 2 and 3. In FIG. 2, an AF evaluation value curve 1 corresponding to the target subject and an AF evaluation value curve 2 corresponding to the person who cuts in front of the target subject set as the focus object are shown.
The AF evaluation value curve 2 corresponding to the person who cuts in front of the target subject set as the focus object is alone a curve of an AF evaluation value having a peak in a position closer to the lens than the subject as indicated by a dotted line in FIG. 2.
When the focus operation is performed in such a situation and AF evaluation values in respective lens positions are acquired, an evaluation value of an image as a whole is as indicated by an AF evaluation value curve 3 indicated by a bold line in FIG. 3. If such a change in an evaluation value curve occurs during the focus operation, an evaluation value acquired by a camera changes from a value on a thin line to a value on a bold line in FIG. 3 or vice versa. Therefore, a peak position of an evaluation value recognized by the camera is different from a correct focus point position for the target subject. As a result, a defocused picture is obtained.
In particular, in self-timer photographing, a person himself/herself as a main subject solely depresses a release button as a shutter. Thus, except photographing of a picture of plural people such as a group picture, the person as the main subject is not present in an angle of view at timing when the release button is half-pressed and the camera performs the focus operation.
Therefore, when a sequence of the focus operation and photographing processing in the self-timer photographing is set as a sequence of the focus operation at a point when the release button is half-pressed and photographing at a point when a fixed time has elapsed after deep-press operation as in the normal photographing, it is difficult to photograph a picture focused on the main subject.
Therefore, in the self-timer photographing, it is desirable to perform the focus operation immediately before photographing when it is expected that the person as the main subject is in the angle of view. As a technique in the past that discloses control of such execution timing of the focus operation, there is JP-A-10-293342. JP-A-10-293342 discloses a technique that allows a user to change timing for performing the focus operation in the self-timer photographing according to the number of people as subjects.
In the self-timer photographing, there is a temporal difference between time when operation of an imaging apparatus is executed and time when photographing is actually performed. Thus, an unexpected person often cuts into a photographing angle of view immediately before the photographing. In this case, a person as an original subject is photographed while being hidden behind another person, resulting in a failed picture.
Even when, regardless of the fact that another person crosses in front of the camera immediately before photographing, the person disappears at an instance of the photographing and the original subject is successfully photographed, since the person crosses in front of the camera during the focus operation, eventually, a defocused picture is obtained.
As a technique for solving such a problem in that another person cuts in an angle of view immediately before photographing, for example, JP-A-6-202222 discloses a method of prohibiting photographing or delaying photographing timing when a photometric value during waiting time of the self-timer photographing changes. JP-A-5-203862 discloses a method of prohibiting photographing when there is a difference between a range finding value at the time of release operation of a shutter button and a range finding value immediately before photographing.
However, in these methods, when a detection error of a change in a photometric value or a misjudgment of measurement of a range finding value occurs, it is likely that photographing is stopped regardless of the fact that the photographing is performed correctly. For example, when a photometric value is used as a guideline, if it is attempted to photograph a person as a main subject with a motion scene of a moving car, a Ferris wheel, or the like in the background, it is likely that photographing is stopped because a change in the photometric value is detected regardless of the fact that the photographing can be performed normally. When a range finding value is used as a guideline, for example, since the range finding value changes at a stage when a photographer who performs release operation enters a photographing angle of view, photographing is stopped against the photographer's will.